In today’s energy-conscious environment, LEDs are often favored over conventional light sources. This is because of their inherent low power and long life. In addition to this, since LEDs are solid-state lighting (SSL), they can be dimmed, allowing the user to create fantastic lighting effects while reducing the overall power consumption.
Obtaining these benefits from LEDs requires an effective LED driver. The LED driver’s effectiveness is linked to its ability to provide an efficient energy source, to ensure LED’s optimal performance and to maintain the long life of LEDs, even both as the driver keeps the LED output intensity constant and while changing intensity. Also, an LED driver that is intelligent and has advanced capability can make lighting solutions even more attractive.
Although an effective LED driver can offer many advantages, there are also challenges in its implementation. This article will show how an 8-bit microcontroller (MCU) can be used to alleviate design challenges and create high-performance LED driving solutions with capabilities beyond that of traditional solutions.
Some 8-bit microcontrollers have building blocks that can create an effective LED driver. The microcontroller in Figure 1 independently controls up to four different colors of LED channels. Each channel has the LED dimming engines created out of the peripherals available in the microcontroller. Each of these engines has an independent closed loop that can control the power converter with minimal to no central processing unit (CPU) intervention. This leaves the CPU free to perform other important tasks such as supervisory functions, communications or added intelligence in the system.
Fig. 1: Diagram of four LED strings being controlled by a Microchip PIC16F1779 8-bit microcontroller
LED Dimming Engine
The current-mode boost converter shown in Figure 2 is an effective LED driver which is primarily controlled by the LED dimming engine. The engine is mainly composed of core independent peripherals (CIP) such as complimentary output generator (COG), digital signal modulator (DSM), comparator, programmable ramp generator (PRG), op amp (OPA) and pulse-width modulator 3 (PWM3). Combining these CIPs with other on-chip peripherals, such as fixed-voltage regulators (FVR), digital-to-analog converters (DAC) and Capture/Compare/PWM (CCP), completes the whole engine. The COG provides the high frequency switching pulse to MOSFET Q1 to allow the transfer of energy and supply current to the LED string. The switching period of the COG output is set by the CCP and the duty cycle, which maintains the LED constant current and is dictated by the comparator output. The comparator produces an output pulse whenever the voltage across Rsense1 exceeds the output of PRG module. The PRG, whose input is derived from OPA output in the feedback circuit, is configured as a slope compensator to counteract the effect of inherent subharmonic oscillation when the duty cycle is greater than 50%.
The OPA module is implemented as an error amplifier (EA) with a Type II compensator configuration. The FVR is used as the DAC input to provide voltage reference to the OPA non-inverting input based on the LED constant current specification.
Fig. 2: LED driver solutions using the LED dimming engine.